# Problem with array

I'm having some errors while copmiling the following:

\begin{equation}
k_{t+1}=\left\{
\begin{array}{l}
$A.s.k_{t}^{\alpha}+(1-\delta).b_{t}^{P}-\delta.b_{t}$ \quad \text{if $\frac{b_{t}+b_{t}^{P}}{(1-\epsilon).s.k_{t}^{\alpha}} < 1$} \\
$s.k_{t}^{\alpha}-b_{t}$ \quad \text{if $\frac{b_{t}+b_{t}^{P}}{(1-\epsilon).s.k_{t}^{\alpha}} \geq 1$}
\end{array}
\right.
\end{equation}


It looks as I want in pdf but I'm getting some missing inserted. Thanks for your help • When I use cases instead of array, I get the error extra alignment tab. \begin{equation} k_{t+1}=\begin{cases}{A.s.k_{t}^{\alpha}+(1-\delta).b_{t}^{P}-\delta.b_{t} & \quad if\frac{b_{t}+b_{t}^{P}}{(1-\epsilon).s.k_{t}^{\alpha}} < 1$& \\ s.k_{t}^{\alpha}-b_{t} & \quad if$\frac{b_{t}+b_{t}^{P}}{(1-\epsilon).s.k_{t}^{\alpha}} \geq 1$} \end{cases} \end{equation} – Bob May 21 '18 at 23:27 • Have also addressed your cases comment in the answer. – Peter Grill May 21 '18 at 23:57 ## 1 Answer The array environment is already in math mode, so you don't need the $ there. Following does not produce the Missing $inserted. ## Notes: • Not sure what the . means in your equations, but did you perhaps intend to use a \cdot? • Your cases example in the comments had an extra & at the end. I have added that example in the MWE. ## Code: \documentclass{article} \usepackage{amsmath} \begin{document} \begin{equation} k_{t+1}=\left\{ \begin{array}{l} A.s.k_{t}^{\alpha}+(1-\delta).b_{t}^{P}-\delta.b_{t} \quad \text{if$\frac{b_{t}+b_{t}^{P}}{(1-\epsilon).s.k_{t}^{\alpha}} < 1$} \\ s.k_{t}^{\alpha}-b_{t} \quad \text{if$\frac{b_{t}+b_{t}^{P}}{(1-\epsilon).s.k_{t}^{\alpha}} \geq 1\$}
\end{array}
\right.
\end{equation}

\begin{equation}
k_{t+1}=
\begin{cases}
A.s.k_{t}^{\alpha}+(1-\delta).b_{t}^{P}-\delta.b_{t}
& \quad \text{if } \frac{b_{t}+b_{t}^{P}}{(1-\epsilon).s.k_{t}^{\alpha}} < 1
\\
s.k_{t}^{\alpha}-b_{t}
& \quad \text{if } \frac{b_{t}+b_{t}^{P}}{(1-\epsilon).s.k_{t}^{\alpha}} \geq 1
\end{cases}
\end{equation}
\end{document}